1. Field of the Invention
This invention relates to devices for use at microwave frequencies, in particular, high-temperature superconducting devices for use at microwave frequencies and, more particularly, to high-temperature superconducting microwave delay lines.
2. Description of the Prior Art
Microwave delay lines are of interest as memory elements in radar, electronic warfare, and communications systems in order to temporarily store signals while they are being pre-processed in other parts of the system. After pre-processing, the stored signals can be properly redirected for further processing. Typically, delays on the order of about 100 to 300 ns are needed for this type of application. Currently available delay lines are typically lengths of coaxial cable with at least one amplifier section. These delay lines limit the dynamic range of the system receiver because of the amplifiers, and are physically large. Delay lines are also needed in satellite communications transponders, where memory storage of signals is required while switching to appropriate channels occurs. Microwave delay lines can be fabricated using a number of planar configurations including the microstrip line structure, the coplanar line structure, and the stripline structure. Although it is important to provide as much delay, i.e., delay line length, as is reasonably possible in an area as small as possible, it also is important that crosstalk does not occur between adjacent delay line segments of the same delay line.
In a pure TEM structure, the even and odd phase velocities of adjacent, parallel coupled lines are the same. Only backward coupling can take place, and this can be controlled by adjusting the distance between the delay lines because it is periodic in the length of the coupler from zero to a maximum determined by the distance between the lines. Microstrip line has unequal, even and odd phase velocities which result in both backward and forward coupling between parallel lines. This is more difficult to control because forward coupling is cumulative with line length. A substantially complete transfer of energy is possible if two loosely coupled lines run parallel to each other for a sufficiently long distance. A coplanar line structure has an inherent shielding between adjacent lines, and, therefore, allows better isolation between these lines. However, it requires that the ground planes be equalized with periodic crossovers which bridge over the signal carrying line. The use of crossovers increases insertion losses and, because they typically are fabricated with gold ribbons, can make fabrication costly.
Some superconducting stripline delay lines have been fabricated for analog signal processing purposes. However, these delay lines were in the form of dispersive delay lines. The delay lines subject of this invention are non-dispersive, i.e., they have constant delay versus frequency over a wide bandwidth, typically 2 GHz or more. Others have fabricated high-temperature superconductive non-dispersive stripline delay lines with a spiral configuration but have not provided composite delay line structures as provided in the invention herein.
What is needed then is a high-temperature superconductive microwave delay line that operates in an essentially pure TEM field configuration in a compact assembly.